WO1994006849A1

WO1994006849A1 - Paper-like film and method and compositions for making it

Info

Publication number: WO1994006849A1
Application number: PCT/CA1993/000385
Authority: WO
Inventors: Kevin Bergevin; Dean Scott Gray
Original assignee: Dupont Canada Inc.
Priority date: 1992-09-17
Filing date: 1993-09-17
Publication date: 1994-03-31
Also published as: GB9219656D0; AU4812493A; MX9305696A

Abstract

A process for making a paper-like film, comprising: a) extruding into film or sheet form of thickness 100 to 2500 νm a composition of i) a polyethylene component of a first high density polyethylene having a melt index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, ii) a filler component of from 5 to 25 wt.% of a lamellar filler, e.g. talc, and from 15 to 50 wt.% of a filler having a low aspect ratio, e.g. calcium carbonate, and iii) up to about 2 wt.% of at least one C10-C24 organic acid based on the total composition, e.g. stearic acid; and b) orienting the film or sheet at least 4 times in at least one direction, the orientation being conducted at a temperature between the line drawing temperature and the melting temperature of the composition, the resulting film having a thickness of from 25 to 200 νm. The total filler component is in an amount of from 30 to 60 wt.%, the percentages being based on the total composition. The weight ratio of the first high density polyethylene to the second high density polyethylene is from 20:80 to 80:20. The blend optionally contains from 0 to 20 wt.%, based on the total polyethylene component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min.

Description

PAPER-LIKE FILM AND METHOD AND COMPOSITIONS FOR MAKING IT The present invention relates to the production of a paper-like thermoplastic film and more particularly to the production of such paper-like films from polyethylene containing inorganic fillers. Such films may be useful for envelopes, posters, packages, books, maps, films for graphic arts applications, labels, in-mould labels and the like.

Paper-like thermoplastic films are known. For example, U.S.Patent 4 082 880, which issued 1978 April 4 to V.G. Zboril discloses paper-like thermoplastic films made from 70-98 wt.% of a polyethylene having a melt index of up to 5 dg/min and 2-30 wt.% of an inorganic lamellar filler, using a blown film process. Canadian Patent 1 043 931, which issued 1978 December 5 to T.M. Tuszynski discloses paper-like thermoplastic butter and margarine wrap made from high density polyethylene having a density in the range of 0.945-0.970 g/cm³ and containing aluminium pigment and, ^•optionally, mica particles. U.S. Patent 3 154 461, assigned to 3M, discloses film made from a selection of polymers and from 1 to 25% filler. The film is biaxially oriented 2.5 to 3.5 times in the transverse and machine directions. U.S. Patent 3 738 904, assigned to E.I. du Pont de Nemours and Company, discloses paper-like films made from blends of homopolymers, copolymers or blends of C₂-Cι₀ ct-olefins having a crystallinity of at least 60%, containing from 26 to

50 wt.% filler. The films had been biaxially oriented at least two times in the machine and transverse directions. It is known in the art that microvoided films can be produced by stretching certain filled polymer compositions. Such film possess certain paper-like qualites, e.g. opacity, whiteness and printability. However, such films suffer from low flexural stiffness. Microscopic evaluation of the internal structure of such films generally reveals a large number of filler particles which are no longer well bonded to the polymer matrix and therefore the particles are not able to provide the flexural stiffness.

One method of providing flexural stiffness involves

SUBSTITUTESHEET coating the film with clay. The clay also makes the films more acceptable for writing or printing thereon. However, coating with clay adds to the expense of the film and makes such films difficult to recycle. The present invention seeks to provide a film which possesses flexural stiffness and may be written or printed on, without the necessity of coating with clay.

Accordingly the invention provides a micro-voided film having a thickness of from 8 to 200 μm, made from a composition comprising i) a polyolefin resin component comprising polyethylene or a blend of polyethylenes, and ii) a filler component consisting of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition, said film having a structure with voids surrounding or adjacent to the low aspect ratio filler particles, in the interior of the film. Preferably the film thickness is from 25 to 200 μm, and especially from 50 to 150 μm.

In one embodiment the polyolefin resin component comprises high density polyethylene or a blend of polyethylenes, at least one of which is a high density polyethylene.

In a preferred embodiment, the polyolefin resin component comprises one or more polyethylenes, at least one of which is a high density polyethylene having a melt index of less than 0.5 dg/min. In a further embodiment, the film is an opaque micro- voided film in which the polyolefin component comprises a blend of a first high density polyethylene having a melt index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, the weight ratio of first high density polyethylene to second high density polyethylene being from 20:80 to 80:20, said blend optionally containing from 0 to 20 wt.%, based on the

SUBSTITUTESHEET total polyolefin component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min, and in which the polyolefin component is blended with a filler component of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition.

In yet another embodiment the opaque micro-voided film contains up to about 2 wt.% at least one Cι₀-C_M organic acid based on the total composition.

In another embodiment the lamellar filler is present in a concentration of from 10 to 20 wt.%

In a further embodiment the lamellar filler is talc. In a preferred embodiment the low aspect ratio filler is present in a concentration of from 25 to 40 wt.%. A preferred low aspect ratio filler is calcium carbonate.

In another embodiment the first high density polyethylene has a melt index of from 0.03 to 0.10 dg/min. In yet another embodiment the second high density polyethylene has a melt index of from 50 to 100 dg/min.

In a further embodiment the low density polyethylene is a linear low density polyethylene having a melt index of from 0.5 to 1.5 dg/min. The film of the present invention is preferably 50 to 150 μm in thickness.

As used herein, melt index is measured by the procedures of A.S.T.M. D-1238-90b, and density of polyethylene is measured by the procedures of A.S.T.M. D- 1505-85. The term polyethylene, as used herein, means ethylene homopolymers or copolymers made of ethylene and at least one other olefin monomer. High density polyethylene means a polyethylene having a density of at least 0.940 g/cm³ and low density polyethylene means a polyethylene having a density of 0.925 g/cm³ or less.

The term "aspect ratio" refers to the ratio of particle length to particle thickness. For any given filler, the

SHEET aspect ratio is the average value determined for a representative number of particles by examination through a microscope. The length is the longest dimension, measured through the centre of mass of the particle. Once the length is known, it is possible to measure the dimensions of the particle in two other directions which are mutually perpendicular to each other and perpendicular to the length. These two dimensions are referred to as the width and thickness of the particle, with the thickness being the smaller of the two, when they are unequal. Fillers with low aspect ratio, i.e. tending to a ratio of 1.0, although irregular, are most often described as spherical, round, cubic. Fillers of high aspect ratio are most often described as lamellar (i.e. plate-like) , fibrous, needle- like. Lamellar fillers are those whose particle lengths and widths are considerably larger than their thicknesses. The present invention also provides a process for making a film, comprising: a) extruding into film or sheet form of thickness 100 to 2500 μm a composition comprising i) a polyolefin resin component comprising polyethylene or a blend of polyethylenes, and ii) a filler component consisting of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition; and b) orienting said film or sheet at a stretch ratio of at least 4 in at least one direction, said orientation being conducted at a film temperature between the line drawing temperature and the melting temperature of the composition, the resulting film having a thickness of from 8 to 200 μm. Preferably the resulting film has a thickness of from 25 to 200 μm, and especially from 50 to 150 μm. In one embodiment the polyolefin resin component comprises high density polyethylene or a blend of polyethylenes, at least one of which is a high density polyethyl

SUBSTITUTESHEET In a preferred embodiment, the polyolefin resin component comprises one or more polyethylenes, at least one of which is a high density polyethylene having a melt index of less than 0.5 dg/min. In another embodiment, in step a) the sheet is extruded to a thickness of from 1000 to 2500 μm.

In one embodiment, the extruded sheet is biaxially oriented, sequentially, first in the machine direction and then in the transverse direction, in a tenter frame. In a further embodiment, the invention provides a process for making a film, comprising: a) extruding into film or sheet form of thickness 100 to 2500 μm a composition comprising i) a polyethylene component of a first high density polyethylene having a melt index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, the weight ratio of first high density polyethylene to second high density polyethylene being from 20:80 to 80:20, said blend optionally containing from 0 to 20 wt.%, based on the total polyethylene component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min, and ii) a filler component of from 5 to 25 wt.% of a lamellar filler and from 15 to 50 wt.% of a filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition, said film or sheet having machine and transverse directions; and b) orienting said film or sheet at a stretch ratio of at least 4 in at least one direction, said orientation being conducted at a film temperature between the line drawing temperature and the melting temperature of the composition, the resulting film having a thickness of from 8 to 200 μm.

In a further_? embodiment the composition includes up to about 2 wt.% of at least one C_I0-C₂₄ organic acid based on the total composition.

In yet another embodiment the sheet is oriented biaxially, at a stretch ratio of at least 4 in each

SUBSTITUTESHEET direction. Preferably the biaxial orientation is in the transverse and machine directions.

In a preferred embodiment the film or sheet is oriented at a stretch ratio of at least 4 in the machine direction and at least 5 in the transverse direction.

In another embodiment the first high density polyethylene has a melt index of from 0.03 to 0.10 dg/min.

In yet another embodiment the second high density polyethylene has a melt index of from 50 to 100 dg/min. In further embodiment the low density polyethylene is a linear low density polyethylene having a melt index of from 0.5 to 1.5 dg/min.

In another embodiment the lamellar filler is present in a concentration of from 10 to 20 wt.% In a further embodiment the lamellar filler is talc.

In a preferred embodiment the low aspect ratio filler is present in a concentration of from 25 to 40 wt.%. A preferred low aspect ratio filler is calcium carbonate.

The resulting film of the present process is preferably from 25 to 200 μm, and especially from 50 to 150 μm in thickness.

The invention also provides a film made by the above process.

The term "line drawing temperature" refers to the temperature above which uniform orientation is obtained, as is known in the art. The line drawing temperature and melting temperature can be determined experimentally.

With respect to the line drawing temperature, when the film is stretched at temperatures low enough for line drawing, a "line" or "neck" develops in the film, perpendicular to the direction of stretch once the yield point has been reached. Stretching then starts from this thinned-out region until an elongation equal to the natural stretch ratio of the film is achieved, for the particular stretch rate used. If a series of film samples is stretched under conditions of line-drawing at a set of increasingly higher temperatures, starting at room temperature, a series

SUBSTITUTESHEET of decreasingly sharp maxima will result in the corresponding stress-strain curves. At some higher temperature, a maximum no longer appears in the stress- strain curve, and line drawing has ceased. At this temperature or higher temperatures, the film undergoes more uniform stretching over its length and no longer displays a line or neck during elongation. A more detailed discussion may be found in "Polyethylene" by Renfrew and Morgan, 2nd edition, pages 170-2, published in 1960 by Interscience Publishers, Inc. New York.

A significant fact related to line-drawing is that the line-drawing temperature can change. For example, a film has a given line drawing temperature before stretching. However, after stretching in one direction, e.g the machine direction, the line-drawing temperature of the film in the direction perpendicular to the direction of stretch, e.g the transverse direction, is higher than the given temperature. This fact should be taken into consideration in order to provide biaxial stretching at the proper stretching temperature.

The melting temperature can be experimentally determined by differential scanning calorimetry.

The term "stretch ratio" is the ratio of a length of a sample of the film in the orientation direction, after stretching, compared to the original length of the film in the orientation direction, before stretching.

During the stretching operation, any of the low aspect ratio particles become detached from the surrounding polymer matrix and small spaces or voids are created around or adjacent to the filler particles. In some cases these voids can remain isolated. In other cases, especially when large numbers of voids are created, these voids can partially merge and become interconnected. Voids which form near the surface of the film may open onto the surface, thereby providing an uneven surface. The term microvoid relates to the small size of the voids, because they are smaller than the film thickness and are generally not visible to the

SUBSTITUTESHEET unaided eye. These microvoids can be observed by fracturing a film specimen and examining the exposed surface or cross- section under an optical or electron microscope.

The presence of microvoids appears to manifest itself as an increase in opacity and whiteness of the film compared to films without microvoids. There is also a noticeable reduction in density which results from the fact that the film is no longer a uniform solid structure. This reduction in density cah create difficulties when comparing film samples of differing degrees of microvoiding. This may be further complicated by films which do not have smooth surfaces. For these reasons, some additional measurements are used for evaluating the films.

The density of a film sample is determined by measuring the length, width and average thickness of the sample and determining its mass. Care must be taken not to overly compress the sample when measuring the thickness. It is desirable to use a micrometer which applies only light force. After the polyolefin and filler components are compounded together, the (compounded) resin density can be determined by using, for example, a density column or other suitable method. "Void fraction" for the film sample is then calculated from the following formula:

Void Fraction = 1 - (Film density/Resin density) It should be noted that the void fraction calculated in this way takes into account internal voids and the effects of surface roughness.

"Equivalent thickness" is calculated as follows: Equivalent thickness = Thickness x (1 - Void fraction) The equivalent thickness is intended to be a measure of the thickness that the film would have had if compressed into a smooth, uniform, solid layer. Except where explicitly stated as "equivalent thickness", the term thickness refers to the measured thickness of the film and not the equivalent thickness.

The invention also provides a composition comprising i) a polyolefin resin component comprising polyethylene or a

SUBSTITUTESHEET blend of polyethylenes, and ii) a filler component consisting of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition, said composition having a structure with voids surrounding or adjacent to the low aspect ratio filler particles, in the interior of the composition. In one embodiment the polyolefin resin component comprises high density polyethylene or a blend of polyethylenes, at least one of which is a high density polyethylene.

In a preferred embodiment, the polyolefin resin component comprises one or more polyethylenes, at least one of which is a high density polyethylene having a melt index of less than 0.5 dg/min.

In one embodiment the composition comprises i) a polyethylene component of a first high density polyethylene having a melt index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, the weight ratio of first high density polyethylene to second high density polyethylene being from 20:80 to 80:20, said blend optionally containing from 0 to 20 wt.%, based on the total polyethylene component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min, and ii) a filler component of from 5 to 25 wt.% of a lamellar filler and from 15 to 50 wt.% of a filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition

In a further embodiment the composition includes up to about 2 wt.% at least one Cι₀-C₂₄ organic acid based on the total composition. In general, the lamellar fillers useful in the present invention have an aspect ratio of at least about 5. Examples of suitable lamellar fillers which may have the

SUBSTITUTESHEET above characteristics are talc, mica and kaolin clay. A preferred lamellar filler is talc.

In general, the low aspect ratio fillers useful in the present invention have an aspect ratio of less than about 3, preferably less than about 2. Examples of suitable low aspect ratio fillers are calcium carbonate, barium sulphate, calcium sulphate, precipitated silica, glass spheres and glass beads. A preferred filler is calcium carbonate.

The particle size of the fillers has an effect on the properties of the film. It is desirable that the fillers do not contain particles of excessively large size, otherwise holes or other defects may be generated during the stretching process. The maximum permissible filler particle size depends on the desired film thickness. A.S.T.M. Procedure D-1210 or sieving may be used to determine the size of the largest particles. When thicker films are to be produced, larger particles can be tolerated. If the average particle size of the low aspect ratio filler is too low, there is a tendency for the resulting films to have lower void fractions.

In general, it is desirable for both the low aspect ratio filler and the lamellar filler to posses a maximum particle size less than about 50 μm. It is also desirable for at least 99.9% by weight of the filler particles to pass through a 325 U.S. mesh screen (nominal mesh openings of

44 μm) . A desirable range for the mean particle size, based on equivalent spherical diameter, for both the low aspect ratio filler and the lamellar filler, is about from 0.2 to 10 μm, preferably from 0.5 to 5 μm. Equivalent spherical diameter (ESD) , which is the diameter computed for a hypothetical sphere which would have the came volume as the particle, may be calculated from:

ESD = (6 x particle volume/π)^% The smallest dimension of the low aspect ratio filler affects the formation of voids in the oriented film.

Voiding is essential for the film of this invention. If the particle size of this filler is too small, the voids are

SUBSTITUTESHEET absent or are too small to give practical paper-like films; if the particle size of this filler is too large, the film tends to have holes therein, thus destroying the integrity of the film. It has been found that the lamellar fillers, in at least some cases, do not substantially contribute to the formation of voids but contribute instead to increased stiffness. Thus, by selecting the appropriate combination of fillers, this invention provides for a micro-voided paper-like film with enhanced stiffness relative to other micro-voided or microporous films.

It has also been found that the smaller the particle size is of the low aspect ratio filler the more organic acid is desirable in order to process the film. Particularly when calcium carbonate is used, it is advantageous to add Cι„ to C₂ organic acid, or blends thereof, to the composition. For certain film applications, the film may be used without further treatment. For other applications, e.g. where the film is to be printed, it is desirable to corona treat the oriented film. For yet other applications, e.g. films for in-mould labels, it may be desirable to corona treat and coat the film with an antistatic agent.

The composition of the present invention is usually first compounded by known methods for melt blending thermoplastic polymers. The compounded composition is then melted in a cast film or blown film extruder and formed into film or sheet, as is known in the art. The resulting film or sheet is typically 200 to 2500 μm thick. This film is then oriented, preferably biaxially, by methods known in the art, e.g. using a tenter frame. The resulting films preferably have a thickness of from 25 to 200 μm.

The optimum temperature for stretching will depend on the particular polyethylene or blend of polyethylenes selected. As indicated, when stretching the films, it is necessary for the film temperature to be below the crystalline melting point and above the line drawing temperature. In practice, the actual sheet or film temperature is not usually measured. Instead, what is

SUBSTITUTESHEET measured is, for example, the temperature of the fluid used to heat orienting rolls in a machine direction orienter, or the air temperature in a tenter frame oven.

If the stretching temperature is too low, the film may break during the stretching operation, or may orient non- uniformly, especially if the film temperature is below the line drawing temperature. Alternatively, when the temperature is too low the lamellar filler particles may initiate voiding and become detached from the polymer, and their beneficial effect on the film stiffness would thus be lost. As the stretching temperature increases, the void fraction decreases. If the film temperature is too high, however, the void fraction may be too low. If the void fraction is too low, a commercially desirable mix of properties may not be achieved.

It is desirable for the films of the present invention to have a void fraction in the range of 0.15 to 0.60, preferably in the range of 0.20 to 0.50.

While the optimum stretching temperatures will depend on the above mentioned factors, they are typically, for a sequential tenter frame stretching process, in the range of 120-130"C for the machine direction orientation (as measured by the heat transfer fluid temperature) and 125-140^βC for the air temperature in the tenter oven. Films of the present invention tend to have low extensibility, high flexural stiffness, good die- cuttability, opacity, fold retention (good deadfold) and printability. As such it is useful as a paper substitute. It is particularly useful, after coating with a heat seal layer, for in-mould labels, as it is recyclable with high density polyethylene resins, containers, films and the like. Heat seal layers are polyolefin materials with a lower melting point thaιn the film of the present invention.

Ethylene vinyl acetate (EVA) copolymers are examples of such heat sealing layers.

It will be understood by those skilled in the art that it may be desirable to include antioxidants, processing aids, UV stabilizers and the like.

The invention is illustrated in the following examples: EXAMPLE I

Filled polymeric compositions were compounded on a Banbury (trade mark) mixer. The compositions (in % by weight) are listed in the following table:

* outside the scope of the present invention.

PE-1 was a high density polyethylene with melt index of 65 and density of 0.959 g/cm³. PE-2 was a high density polyethylene with melt index of 0.055 and density of 0.950 g/cm³. CaC0₃-l was a calcium carbonate with mean particle size of 3μm and typical particle aspect ratio of about 1.5. The talc was a platy talc with a median ESD (equivalent spherical diameter) of 2.2 μm and a typical particle aspect ratio of about 10. The clay was a treated kaolin clay with mean particle size of 0.45 μm, a platelet shape and a typical particle aspect ratio of about 14. The stearic acid blend consisted of 50% Cι₈ (stearic) acid, 42% C_lβ (palmitic) acid, 3% C,₄ acid, 2% C_i7 acid, 2% C_ls (unsaturated acid) and 1% C₁₅ acid.

The compounded resins were extruded in a blown film process to form film having a thickness of 200 μm. Samples of the blown film were placed in the clips of a film stretcher and stretched at a ratio of 4X in the machine direction at a temperature of 133°C.

The flexural stiffness was measured in the machine direction using a Teledyne Taber (trade mark) stiffness tester. The thickness of each sample was determined. A

SUBSTITUTE SHEET void fraction for each group of samples was estimated by weighing a known area of film of measured thickness and comparing the film density to the resin density. For each sample tested, an equivalent thickness was calculated according to the definition given previously.

Because stiffness varies with equivalent thickness, it was only possible to compare the different films by "normalizing" or adjusting the stiffness values to a common thickness basis. The flexural stiffness values were adjusted to the values expected at an equivalent thickness of 63.5 μm. This was done by using a relationship which had been developed previously between stiffness (S) and equivalent thickness (ET) from a large body of data, namely:

S = k * (ET)^2Λ,β so that : S₂/S, = (ET₂/ETι)*-*^lβ The following results were obtained:

* outside the scope of the present invention.

The examples Id, le and If are replicates of each other and serve to illustrate the degree of reproducibility of the technique.

These examples illustrate the beneficial stiffening effects of the lamellar fillers. At the level of 15% talc, there was a substantial stiffening effect, when compared to the talc-free samples. When the level of talc was only 5%, the stiffening effect was less evident. While all of the samples were opaque microvoided films, with thicknesses of 60 to 100 μ , examples la and lc are presented for purposes of comparison only and do not fall within the scope of the present invention. Example lg illustrates the use of clay as an alternative lamellar filler.

SUBSTITUTESHEET EXAMPLE 2

It is advantageous to produce the films of the present invention by a biaxial orientation process, in order to achieve good tear resistance and balanced strength and stiffness (ie) strength and stiffness in both the machine and transverse directions. The following table lists the compositions (in % by weight) and key process parameters which were employed to produce such films. The compositions were prepared by first compounding the raw materials in a Banbury (trade mark) mixer, casting a thick sheet and orienting the sheet sequentially: first in the machine direction and then in the transverse direction.

EXAMPLE 2a 2b 2c 2d

COMPOSITION: PE-1

PE-2

PE-3

CaC0₃-l

CaC0₃-2 TALC

Acid Blend-1

Acid Blend-2

Antioxidant Cast Sheet

Thickness

(μm)

MD* Stretch Ratio

MD Stretch Temperature (*C)

TD* Stretch Ratio

TD Stretch Temperature (^βC)

Film Thickness (μm)

Void Fraction

Equivalent Thicknes

(μm)

MD Stiffness (g.cm)

*TD = transverse direction; MD = machine direction

PE-1 was a high density polyethylene with melt index of 65 and density of¹0.959 g/cm³. PE-2 was a high density polyethylene with melt index of 0.055 and density of 0.950 g/cm³. PE-3 was a linear low density polyethylene with melt index of 0.75 and density of 0.919 g/cm³. CaC0₃-l was a

SUBSTITUTESHEET calcium carbonate with mean particle size of 3 μm and typical particle aspect ratio of about 1.5. CaC0₃-2 was a calcium carbonate with a mean particle size of 1 μm and a typical particle aspect ratio of about 1.5. The talc was a platy talc with median equivalent spherical diameter of 2.2 μm and typical particle aspect ratio of about 10. Acid blend-1 consisted of 50% C_w (stearic) acid, 42% C_lβ (palmitic) acid 3% C₁₄ acid, 2% C_ιr acid, 2% C_lt (unsaturated) acid and 1% C_I5 acid. Acid blend-2 consisted of 52% C_lβ acid, 44% Ciβ acid, 2.5% Cι₇ acid, 1% C_I4 acid and 0.5% C₁₅ acid. The void fractions were determined by measuring the density of the film samples. The flexural stiffness in the machine direction was measured using a Teledyne Taber (trade mark) stiffness tester. The films of example 2 were easy to print and could be written on with pencil or ink. They had excellent fold retention, whiteness, and opacity. Examples 2a, 2b and 2d had excellent tear resistance, strength and stiffness. EXAMPLE 3 Filled polymeric compositions were compounded on a Banbury (trade mark) mixer. The compositions (in % by weight) are listed in the following table:

STEARIC

* outside the scope of the present invention.

PE-4 was a high density polyethylene with melt index of 0.4 and density of 0.945 g/cm³. PE-5 was a medium density polyethylene with melt index of 1.85 and density of 0.935

SUBSTITUTESHEET g/cm³. PE-6 was a high density polyethylene with melt index of 1.0 and density of 0.956 g/cm^3* The talc was a platy talc with median equivalent spherical diameter of 2.2 μm and typical particle aspect ratio of about 1.5. The stearic acid blend consisted of 50% C,₈ (stearic) acid, 42% C_lβ

(palmitic) acid, 3% C₁₄ acid, 2% C_l7 acid, 2% C_lβ (unsaturated acid and 1% Cι₅ acid.

These materials were extruded in a cast film process to produce 100-125 μm films. Samples of these films were placed in the clips of a film stretcher and stretched in the machine direction at a stretching ratio of 4.3X, at the temperatures listed in the table below. The temperatures were chosen so that the samples would become microvoided and therefore somewhat opaque. The resulting films had thicknesses of 25 to 40 μm. For each composition, a set of samples was cut and the density and void fraction were determined. The flexural stiffness of the samples was measured in the machine direction using a Teledyne Taber (trade mark) stiffness tester. In order to compensate for differences in equivalent thickness, the stiffness results were converted to the stiffness (S) expected at an equivalent thickness (ET) of 63.5 μm, again using the formula established previously, namely: s = k * (ET)^2Λlβ

The results are summarized below:

* outside the scope of the present invention. These examples demonstrate the stiffening effect of the talc in the microvoided films. For each of the four polymer compositions, there was an increase in flexural stiffness when part of the calcium carbonate was replaced by talc in the formula. Examples 3a, 3c, 3e and 3g fall within the scope of the present invention. These examples also demonstrate that the flexural stiffness increased as the density of the polyethylene component increased. It is for this reason that high density polyethylene or blends which include high density polyethylenes are preferred.

Furthermore, compositions 3a and 3b were easier to stretch without developing holes or tears and this is attributed to the lower melt index of the polyethylene component. It is for this reason that the inclusion of a low melt index polyethylene is preferred. EXAMPLE 4

The compositions of examples 3a and 3g of Example 3 were pressed into sheets of 250 μm thickness and stretched 4.3X at the temperatures indicated, to produce films with the following properties:

FORMULA STRETCH FINAL VOID FLEXURAL

AS IN: TEMPERATURE THICKNESS FRACTION STIFFNESS

(°C) (μm) (g.cm) 3α 126 56 0.23 0.19 3g 139 61 0.37 0.15

The flexural stiffness of the film samples was measured in the direction in which the samples had been stretched using a Teledyne Taber (trade mark) stiffness tester. The samples were opaque and had good fold retention properties. EXAMPLE 5

SUBSTITUTE SHEET

PE-l was a high density polyethylene with melt index of 65 and density of 0.959 g/cm³. PE-2 was a high density polyethylene with melt index of 0.055 and density of 0.950 g/cm³. The talc was a platy talc with median ESD (equivalent spherical diameter) of 2.2 μm and a typical particle aspect ratio of about 10. CaC0₃-2 was an uncoated calcium carbonate with a mean particle size of 1 μm and a typical particle aspect ratio of about 1.5. CaC0₃-3 was similar but had been pre-coated with stearic acid prior to the compounding step. The stearic acid blend consisted of 50% C_ls (stearic) acid, 42% C_Iβ (palmitic) acid, 3% C₁₄ acid, 2% C₁₇ acid, 2% C_IS (unsaturated) acid and 1% C_i acid.

These compound resins were extruded in a cast film process to produce 100 μm films. Samples of these films were placed in the clips of a film stretcher and stretched in the machine direction at a stretch ratio of 4.3X, at the temperatures listed in the table below. The temperatures were chosen so that the samples would become microvoided and therefore somewhat opaque. The resulting films had thicknesses of about 30 μm. For each composition, a set of samples was cut and the density and void fraction were determined. The flexural stiffness of the samples was measured in the machine direction using a Teledyne Taber (trade mark) stiffness tester. In order to compensate for differences in equivalent gauge, the stiffness results were converted to the stiffness expected at an equivalent thickness of 63.5 μm, again using the formula established previously, namely: S = k * (ET)*^Λlβ

The results are summarized below:

SUBSTITUTESHEET STRETCHING STIFFNESS (g.cm) TEMPERATURE VOID at EQUIVALENT GAUGE

EXAMPLE CC) FRACTION OF 63.5 μm

5a 131 0.24 0.42 5b 131 0.25 0.41

These examples were microvoided and opaque and fall within the scope of the present invention, demonstrating that it is not necessary to add the stearic acid in the compounding step.

SUBSTITUTESHEET

Claims

CLAIMS:

1. A micro-voided film having a thickness of from 8 to 200 μm, made from a composition comprising i) a polyolefin resin component comprising polyethylene or a blend of polyethylenes, and ii) a filler component consisting of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition, said film having a structure with voids surrounding or adjacent to the low aspect ratio filler particles, in the interior of the film.

2. A micro-voided film according to Claim 1 wherein the film thickness is from 25 to 200 μm.

3. A micro-voided film according to Claim 2 wherein the film thickness is from 50 to 150 μm.

4. A micro-voided film according to Claim 1 wherein the polyolefin resin component comprises high density polyethylene or a blend of polyethylenes, at least one of which is a high density polyethylene.

5. A micro-voided film according to Claim 4 wherein the polyolefin resin component comprises one or more polyethylenes, at least one of which is a high density polyethylene having a melt index of less than 0.5 dg/min.

6 An opaque micro-voided film according to Claim 1 wherein the polyolefin component comprises a blend of a first high density polyethylene having a melt index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, the weight ratio of first high density polyethylene to second high density polyethylene being from 20:80 to 80:20, said blend optionally containing from 0 to 20 wt.%, based on the total polyolefin component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min, and in which the polyolefin component is blended with a filler component of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low

SUBSTITUTESHEET aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition.

7. An opaque micro-voided film according to Claim 7 which contains up to about 2 wt.% at least one Cio-C^ organic acid based on the total composition.

8. An opaque micro-voided film according to Claim 6 wherein the lamellar filler is present in a concentration of from 10 to 20 wt.%

9. An opaque micro-voided film according to Claim 6 wherein the lamellar filler is talc.

10. An opaque micro-voided film according to Claim 6 wherein the low aspect ratio filler is present in a concentration of from 25 to 40 wt.%.

11. An opaque micro-voided film according to Claim 10 wherein the low aspect ratio filler is calcium carbonate.

12. An opaque micro-voided film according to Claim 6 wherein the first high density polyethylene has a melt index of from

0.03 to 0.10 dg/min.

13. An opaque micro-voided film according to Claim 6 or

Claim 12 wherein the second high density polyethylene has a melt index of from 50 to 100 dg/min.

14. An opaque micro-voided film according to Claim 7 or Claim 12 or Claim 13 wherein the low density polyethylene is a linear low density polyethylene having a melt index of from 0.5 to 1.5 dg/min.

15. An opaque micro-voided film according to any one of Claims 6 to 13 having a thickness of from 50 to 150 μm.

16. An opaque micro-voided film according to Claim 7 or Claim 12 or Claim 13 wherein the low density polyethylene is a linear low density polyethylene having a melt index of from 0.5 to 1.5 dg/min and the film has a thickness of from 50 to 150 μm.

17. A process for making a film, comprising: a) extruding into film or sheet form of thickness 100 to 2500 μm a composition comprising i) a polyolefin resin component comprising polyethylene or a blend of

SUBSTITUTE SHEET polyethylenes, and ii) a filler component consisting of from 5 to 25 wt.% of at least one lamellar filler and from 15 to 50 wt.% of at least one filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition; and b) orienting said film or sheet at a stretch ratio of least 4 in at least one direction, said orientation being conducted at a film temperature between the line drawing temperature and the melting temperature of the composition, the resulting film having a thickness of from 8 to 200 μm.

18. A process according to Claim 17 wherein the resulting film has a thickness of from 25 to 200 μm.

19. A process according to Claim 18 wherein the polyolefin resin component comprises high density polyethylene or a blend of polyethylenes, at least one of which is a high density polyethylene.

20. A process according to Claim 18 wherein the polyolefin resin component comprises one or more polyethylenes, at least one of which is a high density polyethylene having a melt index of less than 0.5 dg/min.

21. A process according to any one of Claims 17 to 20 wherein in step a) the sheet is extruded to a thickness of from 1000 to 2500 μm.

22. A process according to any one of Claims 17 to 20 wherein the extruded sheet is biaxially oriented, sequentially, first in the machine direction and then in the transverse direction, in a tenter frame.

23. A process according to any one of Claims 17 to 20 wherein in step a) the sheet is extruded to a thickness of from 1000 to 2500 μm, and the extruded sheet is biaxially oriented, sequentially, first in the machine direction and then in the transverse direction, in a tenter frame.

24. A process for making a film, comprising: a) extruding into film or sheet form of thickness 100 to 2500 μm a composition comprising i) a polyethylene component of a first high density polyethylene having a melt

SUBSTITUTESHEET index of less than 0.2 dg/min and a second high density polyethylene having a melt index of at least 30 dg/min, the weight ratio of first high density polyethylene to second high density polyethylene being from 20:80 to 80:20, said blend optionally containing from 0 to 20 wt.%, based on the total polyethylene component weight, of low density polyethylene having a melt index of from 0.4 to 3.0 dg/min, and ii) a filler component of from 5 to 25 wt.% of a lamellar filler and from 15 to 50 wt.% of a filler having a low aspect ratio, and the total filler component being in an amount of from 30 to 60 wt.%, said filler percentages being based on the total composition, said film or sheet having machine and transverse directions; and b) orienting said film or sheet at a stretch ratio of at least 4 in at least one direction, said orientation being conducted at a film temperature between the line drawing temperature and the melting temperature of the composition, the resulting film having a thickness of from 8 to 200 μm.

25. A process according to Claim 24 wherein the composition includes up to about 2 wt.% of at least one

organic acid based on the total composition.

26. A process according to Claim 24 wherein the sheet is oriented biaxially, at a stretch ratio of at least 4 in each direction.

27. A process according to Claim 26 wherein the film or sheet is oriented at a stretch ratio of at least 4 in the machine direction and at least 5 in the transverse direction.

28. A process according to Claim 24 wherein the first high density polyethylene has a melt index of from 0.03 to

0.10 dg/min.

29. A process according to Claim 24 wherein the second high density polyethylene has a melt index of from 50 to

100 dg/min.

30. A process according to Claim 24 wherein the low density polyethylene is a linear low density polyethylene having a melt index of from 0.5 to 1.5 dg/min.

SUBSTITUTESHEET

31. A process according to Claim 24 wherein the lamellar filler is present in a concentration of from 10 to 20 wt.%

32. A process according to Claim 31 wherein the lamellar filler is talc.

33. A process according to Claim 24 wherein the low aspect ratio filler is present in a concentration of from 25 to 40 wt.%. A preferred low aspect ratio filler is calcium carbonate.

34. A process according to any one of Claims 24 to 33 wherein the resulting film from 25 to 200 μm.

35. A film according to any one of Claims 1, 4 or 6 having a void fraction in the range of 0.15 to 0.60.

36. A film according to any one of Claims 1, 4 or 6 having a void fraction in the range of 0.20 to 0.50.

SUBSTITUTESHEET